DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints (2503.14738v2)

Abstract: We present baryon acoustic oscillation (BAO) measurements from more than 14 million galaxies and quasars drawn from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2), based on three years of operation. For cosmology inference, these galaxy measurements are combined with DESI Lyman-$\alpha$ forest BAO results presented in a companion paper. The DR2 BAO results are consistent with DESI DR1 and SDSS, and their distance-redshift relationship matches those from recent compilations of supernovae (SNe) over the same redshift range. The results are well described by a flat $\Lambda$CDM model, but the parameters preferred by BAO are in mild, $2.3\sigma$ tension with those determined from the cosmic microwave background (CMB), although the DESI results are consistent with the acoustic angular scale $\theta_*$ that is well-measured by Planck. This tension is alleviated by dark energy with a time-evolving equation of state parametrized by $w_0$ and $w_a$, which provides a better fit to the data, with a favored solution in the quadrant with $w_0>-1$ and $w_a<0$. This solution is preferred over $\Lambda$CDM at $3.1\sigma$ for the combination of DESI BAO and CMB data. When also including SNe, the preference for a dynamical dark energy model over $\Lambda$CDM ranges from $2.8-4.2\sigma$ depending on which SNe sample is used. We present evidence from other data combinations which also favor the same behavior at high significance. From the combination of DESI and CMB we derive 95% upper limits on the sum of neutrino masses, finding $\sum m_\nu<0.064$ eV assuming $\Lambda$CDM and $\sum m_\nu<0.16$ eV in the $w_0w_a$ model. Unless there is an unknown systematic error associated with one or more datasets, it is clear that $\Lambda$CDM is being challenged by the combination of DESI BAO with other measurements and that dynamical dark energy offers a possible solution.

Overview of DESI DR2. Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

The paper "DESI DR2. Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints" delivers an extensive analysis of Baryon Acoustic Oscillation (BAO) measurements derived from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2). The dataset incorporates redshift observations from over 14 million galaxies and quasars, marking a significant leap in scale compared to prior datasets. The paper aims to place stringent cosmological constraints, primarily focusing on elucidating the expansion history of the Universe and assessing the potential requirement for an evolving dark energy component.

At its core, DESI DR2 enhances the precision of BAO distance measurements which are crucial to our understanding of cosmological parameters such as the matter density parameter Ω_m and the Hubble constant scaled by the sound horizon, h r_d. Under the standard ΛCDM model, the paper reports Ω_m = 0.2975 ± 0.0086 and h r_d = 101.54 ± 0.73 Mpc. While consistent with the constraints obtained from DESI DR1, the increased statistical precision of DR2 exposes a rising tension with the Cosmic Microwave Background (CMB) data from Planck, denoted by a 2.3σ deviation in Ω_m.

A significant contribution of this work lies in broadening the understanding of neutrino mass constraints. By combining DESI BAO data with CMB information, the paper sets an upper limit on the sum of neutrino masses, E m_ν < 0.064 eV, which approaches the lower boundary posited by neutrino oscillation experiments. This stringent bound persists even under various CMB likelihoods and model explorations, underscoring the robustness of this result.

A critical aspect of cosmology addressed by DESI DR2 is the possible evolution of dark energy. Employing the CPL parametrization (w_0waC_DM), the paper presents compelling evidence for a non-constant equation of state w(z). In fits combining DESI, CMB, and Supernovae (SNe) data, evolving dark energy models are favored over the ΛCDM scenario with significance levels ranging from 2.8σ to 4.2σ, depending on the SNe dataset used. Specifically, a model with w_0 > -1 and w_a < 0 is consistently supported, suggesting a potential transition through the phantom line at some redshift above 0.4.

Such analysis does not resolve the 'Hubble tension,' but instead adds layers to the complexity of contemporary cosmological discourse by introducing evidence of discordances within the standard model itself. For ΛCDM to remain viable, the consistent preference across diverse datasets for w_0 and w_a values that indicate time-evolving dark energy cannot be harmonized without novel physics or theoretical adjustments.

Overall, the DESI DR2 BAO analysis extends the precision of cosmological constraints while serving to highlight growing tensions between different cosmological datasets. Despite marshalling unprecedented statistical power in BAO measurements, the paper calls for further exploration into dynamical dark energy models and the integration of complementary observations, like weak lensing from DES, to address the inconsistencies observed.

These findings potentially signal a paradigm shift in cosmological physics or demand the reconsideration and improvement of existing theoretical frameworks to incorporate these high-precision measurements accurately. Future developments may then include even more rigorous constraint techniques and innovative observational strategies to further refine the cosmological model of our universe.